The present invention relates to a motor vehicle with a line pressure control for an automatic transmission, and more particularly to a line presure control during occurrence of a shifting between one gear ratio and another gear ratio in an automatic transmission.
In the automatic transmission, a desired gear ratio is established by hydraulically activating selected one or ones of various friction devices (such as clutches and brakes) by line pressure, and a shift between two gear ratios is effected by changing friction device or devices to be activated
If the line pressure is excessively high, transient engagement capacity of a friction device becomes excessively high, causing great shock to occur, whereas if the line pressure is excessively low, the transient engagement capacity of the friction device becomes excessively low, causing a slip to occur, thus shortening an operating life of the friction device. Thus, the line pressure has to be appropriately controlled. Conventionally, as described in the publication entitled "SERVICE MANUEL FOR AUTOMATIC TRANMISSION OF THE RE4R01A TYPE" (A261C07) published in March 1987 by NISSAN MOTOR COMPANY, LIMITED, a drain circuit of a line pressure regulator valve is opened or closed by a line pressure solenoid of the duty cycle type. The duty varies from 0% to 100%. When the duty is 0%, the line pressure solenoid is left OFF, closing the drain circuit, while when the duty is 100%, the line pressure solenoid is left ON, opening the drain circuit. Thus, increasing the duty causes the line pressure regulator to increase a magnitude of a line pressure generated thereby. Various values of the duty are contained in a table data in a ROM of a microcomputer of an automatic transmission control unit. Different table data are provided as shown in FIG. 5, one (curve A) for use at shifting operation. another (curve B) for normal stable non-shifting operation, for example. The duty values are arranged in each table data as being retrievable by table look-up operation using a variable such as a throttle opening degree.
However, this conventional line pressure control system cannot cope with a sitauation where the line pressure solenoid has a manufacturing variation or the characteristic of the line pressure solenoid degrades with time or a situation where the friction device has a manufacturing variation or the frictional material of the friction device degrades with time. In the former situation, even if the line pressure solenoid is subject to the same duty, the magnitude of line pressure deviates from a target value. In the latter situation, even if the magnitude of line pressure is adjusted to the target value, the friction device does not show a desired performance characteristic. Thus, in any event, the conventoinal line pressure control system fails to avoid occurrence of substantial shift shock or reduction in operating life of the friction device.
When a shift solenoid is switched from ON state to OFF state at a time in a manner to effect a 1-2 upshift of the automatic transmission, a level of pressure for second gear operation increases so as to induce the engagement of a given friction device. This produces a change in a rotational speed ratio of a transmission input shaft to a transmission output shaft from a value corresponding to a first gear to that which corresponds to the second gear. A process of the change in the rotational speed ratio at a low line pressure level is different from that at a high line pressure level. Accordingly, it is possible to determine from a time required for the change in the rotational speed ratio, viz., the duration of an inertia phase, whether the actual line pressure is adequate or not. Thus, a line pressure control system is proposed in the co-pending U.S. patent application Ser. No. 07/289,050, in which the duration of inertia phase is measured during shifting, and the measured duration is compared with a target value. A correction value, which is obtained from the difference between the two, is used for adjusting the line pressure during the next shifting.
With such line pressure control system based on the result from learning, however, if the result from learning is kept employed when one of a group of sensors such as a vehicle speed sensor and a throttle sensor, and an analog-to-digital (A/D) converter malfunctions, the line pressure control based on result from learning may degrade quality of shifting.
The present invention aims at improving a line pressure controlin a motor vehicle such that the result of learning is not employed for line pressure control when one of a group of sensors, and an A/D converter malfunctions.